Following the recent trend of miniaturization, high speed, and unification of various functions of electronic devices, the signal propagation velocity within electronic devices or the signal propagation velocity outside of electronic devices is getting faster.
Accordingly, a printed circuit board using an insulator having a lower dielectric constant and a lower dielectric loss coefficient than prior insulators is becoming necessary. Reflecting this trend, recent attempts to apply a liquid crystalline polymer (LCP), which is an insulator having a lower dielectric constant and that is less influenced by moisture absorption than a prior polyimide, to a flexible printed circuit board have been made.
However, even if the LCP is applied thereto, the degree of improvement is insignificant because the dielectric constant of the LCP (Dk=2.9) is not significantly different from that of polyimide (Dk=3.2), and there is a problem that the compatibility with a PCB preparation process using the prior polyimide in a via hole process using a laser because the LCP has low thermal resistance to cause a problem in a soldering process and has thermal plasticity.
Therefore, there have been efforts to lower the dielectric constant of the polyimide being used as an insulator of prior flexible circuit boards as the solution to the problem. For example, U.S. Pat. No. 4,816,516 disclosed a method of preparing a molded product by mixing a polyimide and a fluoropolymer. However, the patent relates not to an electronic product requiring a low dielectric constant but to a molded product, and it uses a polyimide having a high thermal expansion rate and a low glass transition temperature. Further, it is required to form the polyimide resin into a thin film in order to use the same on a printed circuit board, but the above U.S. patent document does not disclose a copper foil laminate prepared into a thin film.
Furthermore, according to U.S. Pat. No. 7,026,032, a method of lowering the dielectric constant of a product prepared by dispersing fluoropolymer fine powders through a polyimide is disclosed. The above U.S. patent document discloses that the fluoropolymer fine powders are more dispersed in the outer surface than the inner core of the insulator. However, as disclosed in the above U.S. patent document, overall moisture regain of the same can be reduced because the fluoropolymer is plentiful in the outer layer of the insulator and moisture penetration and absorption are lowered because of a fluororesin at the outer surface, but there may be a problem that has not appeared in flexible copper foil laminates composed of the prior polyimide. For example, the polyimide resin disclosed in the above U.S. patent document may become inferior in adhesive strength with a coverlay, a prepreg, or an ACF, the coefficient of thermal expansion (CTE) of the polyimide disclosed in the above U.S. patent document is too large to be applied to a flexible copper foil laminate, and the fluororesin may melt at a temperature of around 380° C. which is applied to the storage process of PCB preparation. There is also a risk that the copper foil circuit may be exfoliated because too much fluororesin exists in the outer surface of the polyimide.
Therefore, it is required to develop a polyimide material having not only a low dielectric constant and a low coefficient of thermal expansion by including a fluororesin in a polyimide, but also having a high elastic modulus and low moisture regain.